Multiple sclerosis (MS), a demyelinating disease associated with neurodegeneration of the CNS, is thought to result from an attack on myelin proteins by autoreactive T helper (Th) cells. Relapsing-remitting (RR) MS is characterized by acute attacks with periods of partial or full recovery between episodes. Current findings suggest that pro-inflammatory Th1 and Th17 cells predominate during relapse and that anti-inflammatory Th2 and T regulatory cells (Tregs) drive remission. Understanding the mechanisms of these shifts is crucial to developing therapies for MS. Previously, our data showed increased expression and activity of the calcium- activated protease calpain in brain, spinal cord, and peripheral blood mononuclear cells (PBMCs) of MS patients during relapse compared to remission. Calpain activates T cells and is involved in the production of Th1 cytokines, while its inhibition promotes upregulation of Th2 cytokines. Calpain also modulates the activity of signaling proteins associated with Th profiles (STATs, NFAT, NFB). Our goal is to determine whether alterations in calpain activity during relapse and remission in MS patients are involved in Th1/Th2 dysregulation via alterations in transcription factors. Preliminary data found increased IL-17 levels in PBMCs isolated from the blood of RR-MS patients, and treating these cells with calpeptin reduced the number of Th17 cells in MBP-activated PBMCs, and inhibited the proliferation of MBP-specific T cells as well. Calpain inhibition also increased the number of myeloid-derived suppressor cells (MDSC) in MS PBMCs. Interestingly, STAT6, which specifically induces Th2 cytokine gene expression, was degraded in activated PBMCs from MS patients, and incubation with calpain inhibitor reversed the degradation, suggesting that calpain inhibition may promote a Th2 profile by reducing STAT6 degradation. Incubating primary neuron cultures with supernatant from activated PBMCs isolated from patients increased neuronal death, which was blocked by calpain inhibitor treatment. Based on these data, we hypothesize that calpain inhibition dysregulates Th cells via alterations of transcription factors and myeloid-derived suppressor cells in MS, and reduces production of inflammatory mediators released into the supernatant of Th1/Th17 cells that induce cell death. Three specific aims will be used to examine the hypothesis: (1) examine the effects of calpain inhibition on Th and Treg cytokine/chemokine profiles and alteration of MDSCs in MS patients; (2) determine the signaling mechanisms responsible for these shifts following calpain inhibition; and (3) identify which cytokines/chemokines secreted by activated Th cells are responsible for cell death or increase protection of neurons in vitro and examine the effects of calpain inhibition on the secretion of these factors. These studies are crucial to identify the mechanisms by which calpain inhibitor therapy may potentially treat MS.